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Search for $α$ decay of $^{151}$Eu to the first excited level of $^{147}$Pm using underground $γ$-ray spectrometry

The alpha decay of $^{151}$Eu to the first excited level of $^{147}$Pm ($J^π= 5/2^+$, $E_{exc}=91.1$ keV) was searched for at the HADES underground laboratory ($\approx 500$ m w.e.). A sample of high purity europium oxide with mass of 303 g and a natural isotopic composition has been measured over 2232.8 h with a high energy resolution ultra-low background n-type semi-planar HPGe detector (40 cm$^3$) with sub-micron deadlayer. The new improved half-life limit has been set as $T_{1/2} \geq 3.7\times 10^{18}$ yr at 68% C.L. Possibilities to improve the sensitivity of the experiment, which is already near the theoretical predictions, are discussed. New half-life limit for $α$ decay of $^{153}$Eu is also set as $T_{1/2} \geq 5.5\times 10^{17}$ yr.